The present invention relates to a light beam scanning lens and a light beam scanning device employing such a light beam scanning lens.
Light beam scanning devices are widely used for writing optical information or reading documents. A focusing lens used in a light beam scanning device for focusing a deflected light beam onto a scanned surface is usually an f.theta. lens. For making light beam scanning devices, there is a demand for an f.theta. lens having a large total deflection angle. If an f.theta. lens comprises two or three lens elements, the total deflection angle exceeds 100.degree., but the cost of the f.theta. lens is high and so is the light beam scanning device employing the f.theta. lens. A single-element f.theta. lens is also proposed for a lower cost (see Japanese Patent Publication No. 61-48684, for example). Japanese Laid-Open Patent Publication No. 54-87540 discloses a lens system having the same function as an f.theta. lens for focusing a divergent light beam incident upon the lens system.
If a light beam scanning lens develops a large curvature of field, then the diameter of a beam spot which scans a surface varies as the scanning position moves, thus failing to scan the surface as desired.
One conventional method of correcting curvature of field employs a curved elongate cylindrical lens (see U.S. Pat. Nos. 4,496,209 and 4,639,072 and Japanese Laid-Open Patent Publication No. 58-200214, for example).
The f.theta. lens proposed in Japanese Patent Publication No. 61-48684 produces a curvature of field of more than 5-10 mm at a half angle of view of about 21.degree. with respect to the focal length f=300 mm.
The lens system disclosed in Japanese Laid-Open Patent Publication No. 54-87540 generates a curvature of field of more than 5 mm at a half angle of view ranging from 15.degree. to 21.degree. with respect to the focal length f=150 mm.
These conventional lens systems are therefore not of sufficient performance for use in a light beam scanning device having a large deflection angle.
The curved cylindrical lens referred to above for correcting curvature of field is not effective in correcting curvature of field in a main scanning direction since the cylindrical lens has no substantial power in the main scanning direction.